Pump runs but nothing comes out, pressure that cycles constantly, one fixture dead while others work, a leak you can hear but can't find — this guide walks through the water system the way a technician does: symptom by symptom, component by component.
The water systems lesson covered how fresh water moves through your RV — two pressure sources, the pump, the valves, and where things connect. This guide is the diagnostic layer: what to check when something isn't working, how to isolate a problem to a specific component, and how to tell a simple fix from one that needs a shop.
Holding tank filling on its own?
That's a specific problem with its own guide. It's almost always a stuck check valve in the pump — not a failed part. Read the holding tank guide first →
Specific test procedures, the exact readings you should see, and what it means when something's off. The kind of detail that tells you whether this is a $12 fix or a call to your dealer.
And you won't be working through it alone. A diagnostic chatbot built on 21 years of real field experience is coming soon — so when you hit a wall, you can ask the question directly.
The pump is a positive-displacement device — it pulls water from the fresh tank and pushes it into the system. Most RV pumps are Shurflo or Flojet diaphragm pumps. They self-prime (pull water up from the tank without needing to be pre-filled), run on 12V DC, and cycle on and off to maintain pressure. They should run only when a faucet is open or pressure in the system drops.
This is a pressure leak — somewhere in the system, water is escaping and pressure is dropping enough to trigger the pump. The pump kicks on to rebuild pressure, rebuilds it, shuts off, pressure drops again, repeat. This is one of the more useful diagnostic symptoms because it confirms there's a leak — you just have to find it.
To locate the leak: Turn on the pump, let pressure build, then shut the pump off at the switch. Watch and listen. A faucet dripping = dripping faucet (not a system leak). Water appearing under a cabinet or in a bay = pipe or fitting leak. Toilet running = toilet seal or valve. Water heater pressure relief valve weeping = overpressure or failing PRV. The leak will identify itself if you look while the system is pressurized and the pump is off.
Confirm 12V is reaching the pump. With the pump switch on, measure DC voltage at the pump's power terminals. Should read 12V or close to it. No voltage = problem is in the switch, fuse, or wiring — not the pump. Voltage present but pump doesn't run = pump motor or internal switch has failed. Most diaphragm pumps are not internally serviceable — replacement is the fix.
The pressure regulator on your city water inlet limits incoming pressure to a safe range for your RV's plumbing — typically 40–60 PSI. Campground pedestals can deliver 80–100 PSI, which will blow fittings, split lines, and damage valves over time. A failed or missing regulator is a common source of slow leaks that develop after a season of camping on high-pressure sites.
To test the regulator: Connect a water pressure gauge (inexpensive, available at hardware stores) between the city water hose and the regulator inlet. Note pedestal pressure. Then connect the gauge after the regulator. The output should read 40–60 PSI regardless of input. If output matches input, the regulator has failed open. If output reads zero or near-zero with water flowing, the regulator is stuck closed or clogged.
Regulators are consumable items. A regulator more than 3–4 seasons old that you haven't tested should be replaced as preventive maintenance — they're inexpensive and the consequence of failure is expensive. Always carry a spare.
If you've been connecting to city water without a regulator — or with one that's failed open — inspect all compression fittings under sinks and behind appliances. High pressure causes slow weeping at fittings that appears as mineral staining or slight dampness. Address these before they become active leaks.
When you suspect a leak but can't locate it by sight, a leak-down test identifies the zone. It works on both pump-side and city-water-side plumbing.
Most plumbing leaks in RVs are at fittings, not in the middle of a line. Push-fit (PEX) fittings that weren't fully seated, barbed fittings where the hose clamp has loosened, and threaded fittings where the thread sealant has degraded are the most common failure points.
When one faucet or fixture has no flow while the rest of the system works normally, the problem is almost always local to that fixture — not a system-wide issue.
Inlet screen clogged: Most faucets have a small aerator screen at the tip of the spout. Unscrew it and check for sediment or debris. This is the first thing to check and the most common cause of reduced flow at a single fixture.
Valve partly closed: Each fixture typically has a shutoff valve underneath. If it was bumped, adjusted, or partially closed during winterization, it restricts flow. Confirm both the hot and cold shutoffs are fully open.
Flex line kinked: The braided flex lines connecting supply valves to the faucet can develop kinks, especially if something was stored under the sink. A kinked line looks fine from outside but has a collapsed inner liner. Disconnect and inspect — replace if kinked.
Cartridge failure: Faucet cartridges (the internal valve mechanism) can fail in a partially-closed position. If flow is low even with screens clean and valves open, the cartridge may need replacement. Most RV faucets use standard cartridges available at plumbing suppliers.